The present invention relates to a fuel cell system, which can be started up under low temperature condition and a method for scavenging the fuel cell system.
As an example of a fuel cell mounted on an electric vehicle, a fuel cell categorized as a type of Proton Exchange Membrane (PEM) has been generally known, which employs solid polymers as electrolytic membranes. The fuel cell of this type has stacked layers of cells, each of which has an anode and a cathode interposing an electrolytic membrane. When a hydrogen gas and air (oxygen gas) are supplied to the anode and cathode, respectively, they cause an electrochemical reaction, generating power and water.
It is known that when power generation by a fuel cell is terminated, water remains in lines and the fuel cell. If a system including the fuel cell is terminated without taking care of the water, the water will freeze under low temperature condition, such as a cold district or a winter season, which invites a problem that performance in terms of low temperature start-up deteriorates. As one of countermeasures for this problem, it is known that air is introduced into both anode and cathode sides so as to conduct scavenging during a termination of a fuel cell system (see patent document No. 1).
Patent document No1: Examined patent application publication H04-33112 (right side of page 1 line 6 from the bottom)
In a conventional fuel cell system, scavenging is carried out immediately after power generation is terminated. This leads to a problem that a driver experiences uncomfortable feeling due to noise, which continues even after ignition is turned off. In view of this problem, it has been proposed that scavenging should be started to carry out when the fuel cell has reached a given condition, where its temperature has approached some low temperature, for example. In this case, scavenging is carried out for a cathode by supplying a constant amount of air, which is determined in advance, irrespective of hydrogen concentration on the anode side.
However, there has been a problem that an amount of air more than necessary is supplied for scavenging a hydrogen gas remaining in an anode even if its amount is small, so that extravagant power is consumed to drive an air supplier. This is ascribed to the fact that a percentage of the hydrogen gas permeating through an electrolytic membrane from the anode to cathode depends on how long it has elapsed after a termination of the fuel cell system. If scavenging is carried out with a constant amount of supplied air, it is necessary that this amount should be determined so as to cope with a condition where a large amount of hydrogen gas remains in the anode.